Abstract Pneumocystis carinii (PC) pneumonia remains a serious complication of HIV infection and other immunocompromised states. The well-known inverse relationship between CD4+ lymphocyte count and the risk of PC infection does not hold all of the answers to mechanisms of host defense against this infection. Therefore, we have been seeking to better understand how CD4+ T cell independent (CD4IND) host defense mechanisms might defend against PC. During the prior funding period we demonstrated that genetic engineering of dendritic cells with adenovirus encoding murine CD40L pulsed with PC antigen robust protective anti-PC antibody responses in both wild-type mice as well as in CD4+ T-cell depleted mice. These data support the concept that therapeutic vaccination to prevent PCP can be achieved in a CD4-independnet (CD4IND) manner. Adoptive transfer of either B-cells or antibodies induced by the DC vaccine confer protection to scid mice demonstrating the key role of antibodies in this protection Moreover this serum reacts strongly with the immunodominant antigen Kex1, as surface expressed subtilisin protease. We identified that the antibody generated with this DC vaccination procedure can be successfully used to identify vaccine candidates for PC using immunoprecipitation and proteomics as well as augment Fc mediated opsonic phagocytosis and killing of the organism in vitro. Furthermore we identified dectin-1 as a key molecule for non-opsonic phagocytosis of PC and created novel Dectin-1 Fc fusions that have both direct PC killing in vitro as well as augmenting FC mediated phagocytosis in vitro. Lastly we have shown that anti-fungal carbohydrate antibodies are part of the natural antibody repertoire in mice and preliminary studies show that these antibodies participate in early clearance of the organism from the lung as well as regulate adaptive immune responses to the organism. Taken together these studies strongly support the hypothesis that antibodies directed against peptide and/or carbohydrate antigens of PC can be developed to prevent or treat PCP. We will test this hypothesis with the following Specific Aims: 1. If our hypotheses are correct then anti-carbohydrate antibodies are part of the natural antibody repertoire in mice and mediate early clearance as well as control antigen uptake in draining lymph nodes (DLNs) of PC challenged mice. 2. Our hypothesis also predicts that anti-carbohydrate IgGs can mediate direct killing of PC in vitro as well as achieve organism clearance in vivo independent of CD4+ T-cells in vitro. 3. Test the hypothesis that the combination of anti-Kex1 and anti-carbohydrate antibodies can mediate primary prophylaxis or treatment of PCP in T-cell deficient mice.